To aid in the laboratory assessment of teratogens and the understanding of the molecular mechanisms underlying teratogenesis, an in vitro culture system for rat embryos has been developed. Rat conceptuses of pregnancy day 10 can be grown continuously for 96 hrs with extensive differentiation of major organs. Utilizing this embryo culture, nutritional and hormonal requirements for embryonic development are being studied, and the system's predictiveness for chemical teratogens is being tested. A microsomal enzyme activating system has been coupled to the embryo culture system which allows the detection of indirect acting teratogens.